All satellite navigation systems rely on very stable satellite clock performances to allow accurate clock behavior prediction. Through dedicated clock parameters, that are estimated on the ground based on measurements over long intervals (e.g., 1-2 days) and are transmitted to a user system via the ranging signal, the user system is able to predict such clock behavior and to use this information for its positioning solution. However, unpredictable events cannot be modeled and thus compensated at user level, and degrade directly the achievable ranging accuracy, since additional biases due to such events need to be considered. In particular, early test results of the planned European satellite navigation system Galileo as well as factory tests have shown that Rubidium clocks, which are already used in the Galileo test satellites (GIOVE-A/-B) and which will be used in 10V (In Orbit Validation) and FOC (Full Operational Capability), are affected by unpredictable frequency jumps, typically at a rate of 1-2 events per month. Such jumps affect the ranging accuracy around 1 meter to 10 meters, which has a major impact on all Galileo services.
For typical positioning services, such as the Open Service (OS) of Galileo this effect is less critical, since not all users are always affected, and jumps also occur only from time to time. Therefore the effect can be compensated, or at least mitigated, by averaging over Galileo's system lifetime (i.e., 20 years). It will, nevertheless, degrade the OS performance.
However, for users of Galileo's integrity system, such as Safety-of-Life (SOL) and Public Regulated Service (PRS) users, such averaged compensation is not possible, since a certain accuracy of the individual ranging signal must be ensured with very high degree of confidence. Thus, all integrity information for each satellite, and all of the time, would need to be inflated by the worst-case effect that might occur according to clock frequency jumps. This would destroy any integrity performance, and would therefore jeopardize the related major Galileo services.
If warnings were broadcast to all users not to consider signals that are affected by frequency jumps, it would improve dramatically the situation, and would allow for feasible Galileo integrity service performances again. It would, of course, still affect the availability of the service, since satellites would be flagged as ‘don't use’ from time to time, and continuity event rate might be increased, but the problem is transferred from a critical integrity problem (wrong information sent to the user that might cause wrong positioning solutions and could lead to hazardous events) to a pure availability problem, which is not safety critical. Therefore, frequency jumps must be detected, and a warning sent to the user to ensure the Galileo integrity services.
Due to the relatively small errors that are imposed by frequency jumps (on the order of a few meters), the typical integrity monitoring that is done on the ground is hardly able to detect such frequency jumps and cannot inform the integrity user accordingly. A method to properly detect the satellite frequency jumps, to provide related information to the user, and to properly apply the information, is necessary to ensure a feasible Galileo integrity service accordingly.